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Full text of "Handbook Of Chemical Engineering - I"

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420                              CHEMICAL ENGINEERING
first increase, then decrease, become zero and change sign as the temperature increases. Obvious desirable properties for a thermocouple are: (1) Capability for resisting corrosion and oxidation. (2) The development of relatively large electromotive forces. (3) A temperature-electromotive force relation such that the electromotive force increases continuously with increasing temperature over the temperature range employed.
For general work at higher temperatures, several different types of couples are employed in this country. Up to 360C. for extreme precision or to 500C. for a precision of 5 to 10C. the couple may consist of one wire of copper and the other wire of constantan. Iron-constantan or nichrome-constantan may be employed for technical processes below 900C. For operation below 1,100C. special patented alloys of chromium and nickel and of aluminum and nickel, "chrom el-alum el" or "nichrome-alumel" are very satisfactory even for continuous service. For the temperature range 300 to 1,500C. the Le Chatelier couple should be employed. This couple consists of one wire of platinum and the other wire an alloy containing 90 per cent platinum and 10 per cent rhodium. Other alloys and metals may be employed for special work but the above combinations are sufficient for almost all technical processes carried on in the temperature range 0 to 1,500C. No satisfactory couple has been developed for operation much above 1,500C.
Constancy of Calibration.A further essential property of a thermocouple is constancy of calibration. Alterations in the calibration may be due to various causes. Inhomogeneity may develop on account of contamination by fumes from the furnace, metallic vapors, by oxidation, etc. Contamination may be usually prevented by the use of suitable protecting tubes surrounding the couple; and the effect of contamination may be minimized by using wire of large cross-section. Different furnace conditions and different types of couples require different methods of protection against contamination. Each process must be considered individually. For example a platinum couple is usually protected against contamination by means of refractory porcelain tubes, but if the atmosphere surrounding the platinum is reducing, the use of porcelain may do more harm than good. In this case, the reducing atmosphere changes the silica of the porcelain into silicon which readily attacks the platinum. The electromotive force of some couples gradually decrease with use. The platinum-platinum 90 per cent, iridium 10 per cent, couple has not proven very satisfactory for this reason, although it develops a much larger electromotive force than the platinum-rhodium thermocouple. The iridium gradually distills from the alloy wire, especially above 1;200C., requiring frequent recalibration of the couple.
Reproducibility of Couples.When thermocouples are employed in the laboratory for scientific purposes, although desirable, it is not of serious importance that the calibration or temperature-electromotive force relation of couples of the same type be exactly similar. However, in the industrial plant this question of reproducibility is of considerable moment. The indicating instruments are usually graduated in degrees of temperature and the graduation applies for one definite temperature-electromotive force relation only. If the temperature electromotive force relations of various couples of the same type are not similar, corrections must be applied to the readings of the indicator, and these corrections will be different for each couple. When several couples are operated with one indicator and when the process is such as to require a frequent renewal of couples, the applying of these corrections becomes very troublesome.